Capacitive touch input devices have proven to be useful in a variety of environments. In a simple configuration, a sensor is a capacitor. In a self-capacitance sensor, a sensor forms a first plate of a capacitor. An electric field forms around the sensor. A user's finger disturbs the electric field as it approaches the sensor. A non-conductive overlay can protect the sensors. As a user's finger approaches the sensor, a change in the capacitance value is detected. In a mutual capacitance sensor, a pair of sensors are in close proximity on a surface to form a capacitor. An electric field forms between the two sensors. As a user's finger approaches the mutual capacitance sensor, the electric field changes and the change can be detected.
In devices that have simple input needs, a matrix of capacitive touch sensors in a panel can provide a good solution. Appliances and toys are examples of devices that may require simple, inexpensive sensors. In another application, remote controls for devices including televisions, audio systems, video players, and cable boxes that use a wireless interface such as Bluetooth, WiFi, infrared, or RF can use touch sensors as inputs. In this type of device, simple scanning of the sensors provides an inexpensive and reliable method of capacitive sensor operation. In this configuration, the sensors connect to access lines in rows and columns. In a detection mode, a scan is made of each row line and each column line to determine the capacitance for each line. This type of scanning can detect a single touch by detecting a change in capacitance. The scanning can be continuous or can activate after a time period elapses, or the scanning can be triggered by some other event such as a motion detector or proximity detector that indicates activity.
It would be desirable to provide for additional input options by allowing touching two different locations on a touch area at one time. In an example where the touch area includes a simple button pattern, each sensor may correspond to a single button. In some examples, the sensors tend to be much smaller than a fingertip and the user “touches” several sensors with a single touch. In either case, the use of a two finger touch is desirable. The ability to detect a two finger touch increases input options without increasing the number of sensors. However, in prior known approaches using row line and column line scanning, when two touches are made to diagonally opposed locations in a capacitive touch matrix, it is not possible to distinguish between the two diagonally opposed options (e.g., top-right and bottom-left vs. top left and bottom right). This problem is called “ghosting.” Ghosting means that when multiple touches are made to two areas in two columns and/or two rows in the matrix, the independent row and column scan approach cannot choose between at least two equally likely solutions. In devices that include additional controller processing power (e.g., cell phones and tablets) and that have more complex touch sensors include the capability to drive a transmit electrode and monitor a separate receive electrode for a single measurement, this advanced hardware and software approach can eliminate ghosting. However, for less expensive devices that do not have these costly arrangements and higher processing power, ghosting remains a problem that needs to be addressed.